(a) Field of the Invention
The present invention relates generally to optoelectronics and, more particularly, to an optoelectronic device based on electric current gating due to resonant interaction of tunneling electrons with optical fields.
(b) Description of Related Art
Optically-gated electrical switches, such as Auston switches, are commonly used in the generation of high frequency terahertz (THz) signals. Optically-gated switches typically include an optical pulse generator (e.g., a laser) and a photoconductor switch mounted on a miniature antenna. When the optical pulse generator is active, it emits an optical field that is focused on the photoconductor switch. The photoconductor switch conducts current in response to the optical field. Rapid switching of the optical pulse generator and the associated switching of the current through the photoconductor switch cause the antenna to emit high frequency signals. The miniature antenna couples the high frequency signal from the photoconductor switch to other circuitry that utilizes or processes the high frequency signal.
The switching speed of an optically-gated electrical switch is limited by the photoconductor switch and not by the optical pulse generator. Optical pulses having 50 femptosecond (fs) pulse widths may be produced by commercially available TI:Al.sub.2 O.sub.3 lasers from various manufacturers (e.g., Coherent Laser Group and Spectra Physics). Optical pulses as short as 6 fs have been experimentally produced using cubic phase compensation. Pulses of this duration correspond to electrical signals having a frequency of approximately 100 THz. However, these switching speeds are not realizable due to the relatively slow response of the photoconductor switches.
Another application of optoelectronic devices is photomixing. Photomixing uses two lasers and a material having non-linear optical properties to generate a signal at the difference frequency. For example, two Ti:Al.sub.2 O.sub.3 lasers may be focused on an epitaxial layer of gallium arsenide (GaAs). The interaction of the two lasers and the GaAs substrate creates a difference frequency signal based on the difference between the laser frequencies. The epitaxial GaAs substrate may be located at the driving point of a miniature antenna that couples the difference frequency signal to other circuitry for processing. Typically, these devices have an output power less than 1 microwatt at 1 THz, and a roll off rate of 12 dB per octave.
A major effort is being made at several laboratories to develop microwave amplifiers based on field emitter arrays (FEA). These devices operate as triodes, in which a gate electrode controls the current. These devices have a unity gain bandwidth of less than 2 GHz because the input is shunted by the gate capacitance. The use of lasers to gate electron emission from a field emitter array (FEA) is also a topic of current research. Current experiments have used a Nd:YAG pulsed laser operating at a wavelength of 1 micrometer(.mu.m) to stimulate electron emission from an array of emitting tips. The emitting tips were made from silicon tantalum disilicide and were coated with gold. The laser beam was focused to a diameter of 3 mm or 4 mm on the emitting tips, with the optical propagation vector in the plane of the tips. The laser's pulse width was 5 nanoseconds (ns). When the power flux density of the laser was 2.7.times.10.sup.11 W/m.sup.2, a pulsed current was emitted from the tips. There was no pulse of current when a lower power flux density was used, and when the power flux density was increased to 4.2.times.10.sup.12 W/m.sup.2 the tips fused due to the intense heat from the process. However, the current pulse was only present when a gold tip coating was used, which suggests that the current pulse may be due to ions caused by field-induced evaporation. The minimum duration for a current pulse obtained in this manner is 2 nanoseconds (ns). This design requires the laser to supply a pulsed power of at least 2 megawatts (MW) to produce a detectable current pulse. This laser power level is not practical for most optical switching and mixing applications.
Accordingly, there is a need for new devices having bandwidths much greater than 1 THz, but this has not been possible with prior configurations. The performance of prior configurations is limited by the magnitude and frequency dependence of the nonlinear response of available materials. The performance of prior configurations based on field emission has also been limited by the use of a triode configuration. Additionally, prior configurations require extremely high optical power to produce detectable current emission.